From Marginal to Central: Marginal Zone‐like B Cells as Critical Targets in Cladribine‐Treated Multiple Sclerosis

OBJECTIVE: Multiple sclerosis (MS) is a chronic autoimmune disease where B cells play a central pathogenic role. Cladribine, an oral therapy, provides durable benefits by reshaping lymphocyte populations, yet its specific long-term impact on distinct B-cell subsets is not fully understood. This study aimed to define cladribine's longitudinal effects on B-cell subsets and characterize their inflammatory properties. METHODS: We conducted a 48-month longitudinal study of 36 persons with relapsing-remitting MS treated with cladribine who had completed 2 annual treatment cycles, using high-parameter flow cytometry to track B-cell subset dynamics. In a parallel cross-sectional analysis, B cells from 16 untreated patients and 16 healthy donors were profiled for polyfunctional cytokine production granulocyte-macrophage colony stimulating factor (GM-CSF), interleukin-6 (IL-6), tumor necrosis factor-α (TNF-α) and key surface markers. RESULTS: Cladribine induced profound, sustained depletion of memory B cells, with marginal zone-like (MZ-like) cells showing the deepest and most persistent loss, alongside reconstitution of transitional and naïve populations. Functionally, MZ-like B cells were the most potent polyfunctional producers of proinflammatory cytokines and immunoglobulin M (IgM), a capacity significantly enhanced in persons with MS compared to healthy donors, and expressed the highest levels of CD1c and CD1d, consistent with specialization in lipid antigen presentation. INTERPRETATION: MZ-like B cells are a dominant, polyfunctional inflammatory subset in MS, equipped for non-canonical lipid antigen presentation and IgM secretion. They represent a preferential pharmacological target of cladribine. Sustained reduction of this pathogenic memory pool, followed by repopulation with less inflammatory B cells, offers a mechanistic basis for cladribine's durable clinical benefit and nominates MZ-like cells as biomarkers of response and therapeutic targets. ANN NEUROL 2026;100:808-823.